1. Field of the Invention
The present invention relates to a display driver IC that drives a display section such as a liquid crystal display panel, and more particularly to a display driver IC having a built-in RAM having display read ports.
2. Description of the Related Art
In a driver having a built-in RAM, data read lines for driving a display section such as a liquid crystal display panel are provided independently of bit line pairs for reading or writing data between the driver and an MPU.
FIG. 7 shows a display read line OLCD that is commonly connected to a plurality of memory cells M11 to Mn1 that are arranged column-wise. Although omitted in FIG. 7, the memory cells M11 to Mn1 are provided with switches (for example, P-type MOS transistors) that are connected to the display read line OLCD. One end of the display read line OLCD is connected to an N-type MOS transistor Tr for pre-charging the display read line OLCD to, for example, a LOW potential before reading data.
With further miniaturization of semiconductor processes achieved in recent years, RAMs having a greater memory capacity can be manufactured. Accordingly, the number n of memory cells n disposed in the column direction shown in FIG. 7 can be increased. With more memory cells, the total length of the display read line OLCD becomes longer, such that its parasitic resistance and parasitic capacity increase. Also, the number of memory cells (more particularly the P-type MOS transistors described above) that are connected to the display read line OLCD increases, which becomes a capacitive load.
When the load of the display read line increases, the pre-charge transistor Tr requires a higher drive current, the power consumption during a pre-charge operation increases, and the pre-charge time also increases. On the other hand, when display data is read out, the large sized pre-charge transistor becomes a load, and therefore the drive current of an output driver that is connected to the display read line OLCD must also be increased. As a result, with an increased memory capacity, the power consumed by the RAMs increases. In this regard, a larger memory capacity implemented in a display driver IC, which is used in portable equipment that needs re-charging such as a mobile phone, can cause a problem.
In view of the above, it is an object of the present invention to provide a display driver IC that overcomes the above-described problems.
It is another object of this invention to provide a display driver IC that can restrain an increase in the power consumption when its memory is provided with a greater capacity.
In accordance with one embodiment of the present invention, a display driver IC comprises: a plurality of memory cells arranged in a column direction and divided into a plurality of blocks, each block containing a specified number of memory cells; a plurality of display word lines that successively select the plurality of memory cells in the column direction; a main read line commonly shared by the plurality of memory cells, which transfers display data read from each one of the memory cells that are successively selected by the plurality of display word lines; a plurality of sub-read lines, one for each column-block of memory cells for transferring display data read from each one of the memory cells in that column-block; a plurality of main switches, one for each column-block for selectively connecting a corresponding one of the sub-read lines to the main read line; and a plurality of sub-switches, one for each of the plurality of memory cells, and selected by a respective one of the display word lines to thereby connect that memory cell to a corresponding one of the sub-read lines.
In the display driver IC of the present invention, the load connected to one main read line is composed of one of the main switches, one of the sub-read lines that is connected to the corresponding main switch, and a plurality of sub-switches that connect to the corresponding sub-read line, such that the load is not dependent on the total number of the memory cells in the column direction. Accordingly, even if the total number of memory cells in the column direction increases as a result of increased memory capacity, the load on the main read line is substantially reduced compared to a conventional structure.
In the present invention, the read path also needs to be pre-charged before a data reading operation. For this, a corresponding pre-charge device may be connected to each of the sub-read lines, or a pre-charge device may be connected to the main read line. Furthermore, the driver may include a pre-charge device for each sub-read line and main read line.
The plurality of display word lines may include a plurality of main word lines, one for each block, for selectively turning on a corresponding one of the main switches. In this case, each of the main word lines may be activated by an upper bit signal of an address signal.
The plurality of display word lines may also include a plurality of sub-word lines in each column-block for selecting a corresponding one of the memory cells in that column-block. In this case, each of the sub-word lines may be activated by a lower bit signal of the address signal.
Each pre-charge device may preferably pre-charge both the main read line and the corresponding one of the sub-word lines during a period in which one of the main switches is turned on and before a period in which one of the sub-switches is turned on. In this way, all of the data read paths could be pre-charged.
The effects and advantages of the present invention are more fully realized when the total number of the plurality of memory cells is 280 or greater. In contrast, when the total number of memories in a conventional structure reach that number, the increase in power consumption makes it difficult to realize a portable apparatus.